Cube will not collide no matter what
You answer was very uninformative and i don't quite understand your issue, so please update your question and i'll update my answer accordingly. 

I'll still attempt to help by saying why it might not collide.

If your bullet is not colliding with things, the reason may be many things, so a quick troubleshoot is to check your physics tab(1) 

One, the collision group(2), if a cube had the top left box ticked (dark grey if ticked) and the floor had it's collision mask(3) on the top left as well they will collide.

And if Ghost(4) is ticked, then the cube will fall no matter what through anything, and can't be interacted with say, the mouse click.